N- and S-doped nanoporous carbon framework derived from conjugated microporous polymers incorporation with ionic liquids for efficient oxygen reduction reaction

https://doi.org/10.1016/j.mtener.2020.100382Get rights and content

Highlights

  • Novel N, S-doped hard carbon frameworks were prepared as metal-free ORR electrocatalysts.

  • It yields a half-wave potential of 0.82 V and onset potential of 0.98 V (vs. RHE) in alkaline medium.

  • It exhibits a high diffusion limiting current density of 4.2 mA cm−2.

  • A high value of n is 3.96 was obtained by RRDE measurements.

  • It shows better methanol immunity than that of commercial 20 wt% Pt/C.

Abstract

The rapid growth of high-performance metal-free electrocatalysts is of great significance for widespread realization of fuel cells since noble metals (e.g. platinum) used as electrocatalysts have long been considered as major bottleneck which limited their commercialization. Herein, we report the creation of a kind of novel porous carbon framework containing dual heteroatoms (N and S, named as C-CMPs-NS), which was fabricated by direct pyrolysis of ionic liquids loaded on conjugated microporous polymers, application of highly efficient metal free electrocatalyst in oxygen reduction reaction (ORR). Due to their large specific surface area, outstanding porosity and, significantly, active sites exposed own to high density doping of S and N, the catalyst showed good oxygen reduction reaction activity in alkaline electrolyte. The C-CMPs-1NP yielded a half-wave potential of 0.82 V, high onset potential (0.98 V vs. RHE), the high diffusion limiting current density of 4.2 mA cm−2 and better methanol tolerance than commercial Pt/C where nearly no ORR polarization curve shift and no change of oxygen reduction peak in cyclic voltammetry (CV) were observed in 3.0 M methanol solution. Based on these merits mentioned above, the C-CMPs-NS may hold great potentials as promising alternative of precious metal catalysts for next generation fuel cells.

Introduction

As the most promising reactions for conversion technologies and electrochemical energy storage-the oxygen reduction reaction (ORR), which includes metal-air batteries and fuel cells [[1], [2], [3]]. Well known, the Pt-based catalysts as the noble metals catalysts have been already extensively regard as high active fractions for the ORR reaction [[4], [5], [6], [7]]. However, owing to the rareness of Pt, resulting in the commercialization of fuel cells with a major obstacle that the high cost of Pt-based catalysts, and Pt have another troublesome problem under actual operating conditions, that is instability. Furthermore, in direct methanol fuel cells (DMFCs), Pt is easily poisoned due to this crossover phenomenon of methanol directly from the anode to the cathode [[8], [9], [10]]. Therefore, there is an urgent need to develop efficient and durable alternatives catalysts which at low cost, ideally with dual functionality for ORR [[11], [12], [13]]. Hence, non-platinum/precious metal catalysts are one of the best choices to solve the above problems [14,15].

Therefore, the structure of noble metal base alloy has been proved to be one of the most effective ways to decrease the noble metal consumption, besides, it can significantly improve the comprehensive catalytic performance of noble metal catalyst in oxygen reduction reaction, including Pt-based nanostructured [16], Pt-based alloy [17] and Pt-lanthanide [18]. Comparatively, the metal-free electrocatalysts or the manufacture of non-noble metal with high performance for ORR is much more attractive with the rapid growth of science and technology makes is possible to prepare metal free electrocatalysts possess superior ORR activity or stability even than that of Pt/C.

To this day, some non-noble metal or metal-free electrocatalysts such as metal-free nitrogen-doped carbon (N-doped C) [[19], [20], [21]], non-precious-metal oxides and carbides [21], [22], [23], transition-metal-coordinating macrocyclic compounds [24,25] and transition-metal-coordinating nitrogen-doped carbon catalysts (M-N/C) [26] have been reported as active catalysts for ORR in the most of the recent studies along this line. In particular, N-doped carbon based electrocatalysts, e.g., Metal-N-C catalysts prepared from Fe–N4 and Co–N4 macrocycles precursors [27], N-doped graphene [28] and CNTs [29], etc., have received intensive attention because of their low cost, superior ORR activity, electrochemical stability and broader availability. The high ORR activity of those N-doped carbon are well known to be generated from the pyridine N bonded on carbon which could offer a strong affinity to oxygen atoms and in turn promotes the ORR [7], while in the case of Metal-N-C catalysts, e.g., the formation of unsaturated iron atom by N and C coordination (Fe–N/C) is an important way to prepare complex ORR catalyst of Fe–N/C [30,31].

In addition to the direct application of carbon based materials to this problem, many natural products [32,33] or synthetic polymers [34] have been used as substrate for production of electrocatalysts or may be conductive support material for electroctatlysts by uncomplicated carbonization add or not add dopants. In most cases, nevertheless, the discrepancy chemical make-up or formation of these mentioned precursors result in the performance of synthetic products is unpredictable or not replicable (e.g. porosity, active sites, accessible pores, etc.) after carbonization, this restricted their practical applications.

Conjugated microporous polymer (CMPs) as a subclass of porous organic polymer (POP), has extended π-conjugated three-dimensional (3D) network. Act as an ideal precursor of porous hard carbon materials, CMPs have a great superiority compared to those bulk polymeric compound on account of their large specific area and excellent porosity could remain after carbonization treatment owing to their rigid aromatically conjugated structure [[35], [36], [37], [38]]. Such carbonized CMPs products with excellent accessible surface area and abundant porosity would be advantageous to fully adsorption of molecular oxygen, fully show of active sites and the fast transportation of oxygen species and electrons. More interestingly, the plentiful surface chemistry of CMPs (e.g., the active carbon carbon triple bond) allows they could easily be tailored chemically (for instance, introduction of N into CMPs network) or strongly bind the metal at atomic level [39], thus makes these excellent precursors for preparation of non-noble metal or metal free ORR electrocatalyst. In a continuation of our previous studies on CMPs, further improved catalytic performance by introducing heteroatoms [40], here we show a novel strategy for the creation of a kind of novel porous carbon framework containing dual heteroatoms (N and S, named as C-CMPs-NS), which was fabricated by direct pyrolysis of ionic liquids loaded on conjugated microporous polymers, for effective electrocatalytic reduction of oxygen. As a proof of concept study, the resulting N- and S-doped porous carbon framework derived from CMPs exhibits good electrocatalytic activity, high stability as well as long-term durability, and excellent methanol tolerance, rending them promising candidate as high efficiency metal-free electrocatalysts for ORR.

Section snippets

Result and discussion

In this work, we synthesized five CMPs-based materials by Sonogashira-Hagihara cross-coupling reaction use 1,3,5-triethynylbenzene, 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide and three different monomers. We have found that the resulting porous network structure is a three-dimensional structure consisting of aromatic nodes produced by trigeminal alkynyl bonding, the simulated structure proves this as shown in Scheme 1a and b. In addition, we found that the obtained

Conclusion

To sum up, we have proved a novel pathway for preparation of N-doped hard carbon as a new type of non-metallic catalyst derived from conjugated microporous polymers for highly efficient oxygen reduction reaction. Due to the special structure of CMPs, the prepared catalyst has super porosity and large specific surface area (782.9 m2g-1), which would greatly promotes the exposure of high active sites and the rapid transport of oxygen electrons. Besides, owning to the lone pair electrons of

Author statement

Ruin Jiao: Conceptualization, Methodology, Validation, Formal analysis, Writing- Original Draft, Visualization. Wanli Zhang: Methodology, Validation, Formal analysis. Hanxue Sun: Writing-Original draft preparation, Investigation, Supervision, Resources. Zhaoqi Zhu: Formal analysis, Data curation, Resources. Zifeng Yang: Methodology, Formal analysis. Weidong Liang: Resources, Project administration. An Li: Conceptualization, Resources, Supervision, Project administration, Funding acquisition,

Declaration of Competing Interest

No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. This work described was original research which has not been published previously, and not under consideration for publication elsewhere.

Acknowledgment

The authors are grateful to the National Natural Science Foundation of China (Grant No. 21975113, 51663012 and 51962018), the Natural Science Foundation of Gansu Province, China (Grant No. 1610RJYA001), Support Program for Hongliu Young Teachers (Q201411), Hongliu Elitist Scholars of LUT (J201401), Support Program for Longyuan Youth, Fundamental Research Funds for the Universities of Gansu Province, Project of Collaborative Innovation Team and Innovation and Entrepreneurship Talent Project of

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